Ink jet printers have printheads that operate a plurality of inkjets that eject liquid ink onto an image receiving member. The ink may be stored in reservoirs located within cartridges installed in the printer. Such ink may be aqueous, oil, solvent-based, or UV curable ink or an ink emulsion. Other inkjet printers receive ink in a solid form and then melt the solid ink to generate liquid ink for ejection onto the imaging member. In these solid ink printers, the solid ink may be in the form of pellets, ink sticks, granules or other shapes. The solid ink pellets or ink sticks are typically placed in an ink loader and delivered through a feed chute or channel to a melting device that melts the ink. The melted ink is then collected in a reservoir and supplied to one or more printheads through a conduit or the like. In other inkjet printers, ink may be supplied in a gel form. The gel is also heated to a predetermined temperature to alter the viscosity of the ink so the ink is suitable for ejection by a printhead.
A typical full width scan inkjet printer uses one or more printheads. Each printhead typically contains an array of individual nozzles for ejecting drops of ink across an open gap to an image receiving member to form an image. The image receiving member may be a continuous web of recording media, a series of media sheets, or the image receiving member may be a rotating surface, such as a print drum or endless belt. Images printed on a rotating surface are later transferred to recording media by mechanical force in a transfix nip formed by the rotating surface and a transfix roller. In an inkjet printhead, individual piezoelectric, thermal, or acoustic actuators generate mechanical forces that expel ink through an orifice from an ink filled conduit in response to an electrical voltage signal, sometimes called a firing signal. The amplitude, or voltage level, of the signals affects the amount of ink ejected in each drop. The firing signal is generated by a printhead controller in accordance with image data. An inkjet printer forms a printed image in accordance with the image data by printing a pattern of individual ink drops at particular locations on the image receiving member. The locations where the ink drops landed are sometimes called “ink drop locations,” “ink drop positions,” or “pixels.” Thus, a printing operation can be viewed as the placement of ink drops on an image receiving member in accordance with image data.
In order for the printed images to correspond closely to the image data, both in terms of fidelity to the image objects and the colors represented by the image data, the printheads must be registered with reference to the imaging surface and with the other printheads in the printer. Registration of printheads is a process in which the printheads are operated to eject ink in a known pattern and then the printed image of the ejected ink is analyzed to determine the orientation of the printhead with reference to the imaging surface and with reference to the other printheads in the printer. Operating the printheads in a printer to eject ink in correspondence with image data presumes that the printheads are level with a width across the image receiving member and that all of the inkjet ejectors in the printhead are operational. The presumptions regarding the orientations of the printheads, however, cannot be assumed, but must be verified. Additionally, if the conditions for proper operation of the printheads cannot be verified, the analysis of the printed image should generate data that can be used either to adjust the printheads so they better conform to the presumed conditions for printing or to compensate for the deviations of the printheads from the presumed conditions.
Analysis of printed images is performed with reference to two directions. “Process direction” refers to the direction in which the image receiving member is moving as the imaging surface passes the printhead to receive the ejected ink and “cross-process direction” refers to the direction across the width of the image receiving member. In order to analyze a printed image, a test pattern needs to be generated so determinations can be made as to whether the inkjets operated to eject ink did, in fact, eject ink and whether the ejected ink landed where the ink would have landed if the printhead was oriented correctly with reference to the image receiving member and the other printheads in the printer. In some printing systems, an image of a printed image is generated by printing the printed image onto media or by transferring the printed image onto media, ejecting the media from the system, and then scanning the image with a flatbed scanner or other known offline imaging device. This method of generating a picture of the printed image suffers from the inability to analyze the printed image in situ and from the inaccuracies imposed by the external scanner. In some printers, a scanner is integrated into the printer and positioned at a location in the printer that enables an image of an ink image to be generated while the image is on media within the printer or while the ink image is on the rotating image member. These integrated scanners typically include one or more illumination sources and a plurality of optical detectors that receive radiation from the illumination source that has been reflected from the image receiving surface. The radiation from the illumination source is usually visible light, but the radiation may be at or beyond either end of the visible light spectrum. If light is reflected by a white imaging surface, the reflected light has a similar spectrum as the illuminating light. In some systems, ink on the imaging surface may absorb a portion of the incident light, which causes the reflected light to have a different spectrum. In addition, some inks may emit radiation in a different wavelength than the illuminating radiation, such as when an ink fluoresces in response to a stimulating radiation. Each optical sensor generates an electrical signal that corresponds to the intensity of the reflected light received by the detector. The electrical signals from the optical detectors may be converted to digital signals by analog/digital converters and provided as digital image data to an image processor.
The environment in which the image data are generated is not pristine. Several sources of noise exist in this scenario and should be addressed in the registration process. For one, alignment of the printheads can deviate from an expected position significantly, especially when different types of imaging surfaces are used or when printheads are replaced. Additionally, not all jets in a printhead remain operational without maintenance. Thus, a need exists to continue to register the heads before maintenance can recover the missing jets. Also, some jets are intermittent, meaning the jet may fire sometimes and not at others. Jets also may not eject ink perpendicularly with respect to the face of the printhead. These off-angle ink drops land at locations other than were they are expected to land. Some printheads are oriented at an angle with respect to the width of the image receiving member. This angle is sometimes known as printhead roll in the art. The image receiving member also contributes noise. Specifically, structure in the image receiving surface and/or colored contaminants in the image receiving surface may be identified as ink drops in the image data and lightly colored inks and weakly performing jets provide ink drops that contrast less starkly with the image receiving member than darkly colored inks or ink drops formed with an appropriate ink drop mass. Thus, improvements in printed images and the analysis of the image data corresponding to the printer images are useful for identifying printhead orientation deviations and printhead characteristics that affect the ejection of ink from a printhead. Moreover, image data analysis that enables correction of printhead issues or compensation for printhead issues is beneficial.
One factor affecting the registration of images printed by different groups of printheads is media shrinkage. Media shrinkage is caused as the media is subjected to relatively high temperatures as the media moves along the relatively long path through the printing system. In a web printing system, any high temperatures can drive moisture content from the web, which causes the web to shrink. If the physical dimensions of the web change after one group of printheads has formed an image in one color ink, but before another group of printheads has formed an image in another color of ink, then the registration of the two images is affected. The change may be sufficient to cause misregistration between ink patterns ejected by the different groups of printheads. The amount of shrinkage depends upon the heat to which the web is subjected, the speed of the web as it moves over heated components, the moisture content of the paper, the type of media material, and other factors.
Media shrinkage affects the accuracy of the image analysis that enables printhead position correction. If media shrinkage is not considered during the analysis, the compensation data generated during the analysis are insufficient to achieve proper registration between the printheads. Reducing the effect of web dimensional changes on the analysis of test pattern images and the generation of the correction data for printhead positioning is a goal in web printing systems.